Accumulation conditions and patterns of natural gas hydrate in the Muli permafrost area, northeastern margin of the Qinghai-Tibet Plateau, NW China

The Muli permafrost area is the only midlatitude permafrost area where natural gas hydrates (NGHs) have been drilled. Although a series of achievements has been made in studies on NGH accumulation conditions in the permafrost area, related studies on gas hydrate stability zones, gas sources and groundwater are still controversial and need further work. Based on geophysical, geological and geochemical data, this paper systematically elaborates the main NGH accumulation conditions in the Muli area in terms of the pressure-temperature stability conditions, fault structure, gas composition, gas source, reservoir characteristics, groundwater distribution and topographic characteristics. The results show that the thicknesses of the NGH stability zone range from 491.0 to 962.7 m. The fault structure can provide a pathway for hydrocarbon gas migration from deep to NGH stability zones, in which thrust faults can prevent the upwards migration of gas by sealing it below the hanging wall block. Rock fractures in the NGH stability zone can not only be gas migration channels, but also serve as a reservoir space for NGH accumulation. The hydrocarbon gas of NGHs shows multiple origins and is mainly composed of oil-associated gas. The formation of frost mounds related to the overpressured gas ejection caused by NGH decomposition may imply the existence of NGHs. Synthetically, similarities and differences in NGH accumulation conditions between the Muli permafrost area and other high-latitude permafrost areas are proposed, and the 'regression' NGH accumulation pattern in the Muli area is constructed. The study results can enrich theoretical knowledge of terrestrial NGH accumulation, and provide scientific guidance for NGH exploration in midlatitude permafrost areas.